1. Field of the Invention
This invention relates to continuous casting of metal sheet in strip form (hereinafter sometimes referred to as strip), and more particularly to an improved method of high speed direct casting of thin metal sheet by withdrawing the sheet as a continuous strip from a supply of molten metal on a chilled casting surface.
2. The Prior Art
The desirability of directly casting metal sheet in thin strip form on a continuous basis has long been recognized, but the production of a commercially acceptable product by such process, or operation of such a process for sustained periods at commercially acceptable speeds generally has not been considered possible. Such a process would result in substantial savings in energy, manpower and equipment over the conventional process of rolling the strip from ingots, slabs and plates. As used in this application, the terms "sheet" and "strip" are intended to mean crystalline metal sheet in continuous strip form having a width of at least 12 inches and preferably 20 inches or greater, and having a thickness within the range of about 0.015 to about 0.080 inches and preferably within the range of about 0.020 to about 0.060 inches.
Efforts to develop a commercially acceptable process for direct casting of metal sheet on a continuous basis have included various arrangements for contacting a melt with a moving chilled casting surface (chill) to solidify and withdraw the cast strip. These efforts have included flowing the melt at a substantially constant rate onto a moving chill for solidification and withdrawing the solidified strip from the chill in a continuous process as shown in British Patent No. 6,630; conducting the melt from a tundish through a restricted outlet so as to provide a convex meniscus which is contacted by the moving chill as shown for example in British Patent No. 20,518 and U.S. Pat. No. 3,522,836; causing the melt to overflow at an edge or wall of a container or tundish to contact a moving chill as shown in U.S. Pat. No. 993,904 and Japanese published application 58-41656; flowing the melt into the nip of a pair of spaced counter-rotating chill rolls to withdraw a strip which is rolled and chilled on both surfaces as shown for example in U.S. Pat. No. 4,212,344; partially submerging a driven cylindrical chill into the melt to withdraw a strip (or filaments) as disclosed in U.S. Pat. Nos. 3,540,517 and 3,817,901; and, partially submerging a pair of counter-rotating cylindrical chills in a melt to withdraw a continuous strip chilled and rolled on both sides as shown for example in U.S. Pat. No. 3,823,762.
In all of the known prior art continuous or direct strip casting processes employing a continuously driven chill which contacts and withdraws molten metal from a melt (hereinafter referred to generally as a melt drag process), the metal is solidified by extracting heat through the chill so that a thin skin is solidified immediately upon contact with the chill. This skin grows in thickness as the chill moves progressively through or past the melt until the strip is formed. Accurate control of heat transfer between the metal being solidified and the chill during this strip solidifying process is critical to forming a uniform product in any of the melt drag systems mentioned above.
The thin skin formed upon initial contact of the melt with the chill is firmly adhered to the chill and this intimate, bonded contact results in a maximum heat transfer from the melt to the chill. As the solidifying skin progressively increases in thickness, the heat extraction results in contraction of the solidifying strip until the bond is broken, thereby resulting in a substantial reduction in the rate of heat extraction.
The production of high quality directly cast strip product depends to a large degree upon a controlled heat transfer rate and uniform release of the cast product from the chilled surface. For example, when the heat transfer rate is too high, release can result before the solidifying skin has attained the desired or necessary thickness and remelting or breakage may result. Non-uniform release will result in gauge variations which may be so great as to make subsequent cold rolling operation difficult or impossible. Also, surface cracks may be formed in the top surface of the product, i.e., the product surface not in contact with the chill.
Various attempts have been made to accurately control the heat transfer rate between the melt and the chill in a direct or continuous melt drag strip casting operation. These attempts have included providing a knurled surface on the chill (U.S. Pat. No. 3,345,738) to thereby impart to the bottom surface of the solidifying strip a network or grid of point indentations. This patent teaches that the point indentations provide controlled or uniform release of the solidified strip from the chill to produce a more uniform cooling rate and strip thickness.
Another attempt to control the heat transfer rate and provide a more uniform release of the strip from the chill is disclosed in U.S. Pat. No. 3,540,517. According to this patent, direct contact between the melt and chill is avoided by applying a thin coating of a particulate refractory material to the chill prior to its contact with the melt. The particulate refractory is applied in a water slurry or other fast drying solution on a continuous basis as by spraying. The slurry may be applied with or without a binder. The particulate material may be alumina, silicate oxide, magnesia or ground fire clay particles, for example, and is intended to break up the contact area between the solidifying melt and the chill. This technique, however, necessarily results in a substantial number of the refractory particles adhering to and/or becoming embedded in the surface of the product and a uniform casting thickness or particle distribution cannot always be achieved.
Other prior art practices for controlling the rate of heat transfer between a melt and a chill in a continuous strip casting process are discussed in U.S. Pat. No. 4,250,950. As stated in this patent, the "state of the art" methods involve either (1) coating the chill with a thermally insulating or protective layer, or (2) mechanically roughening the chill surface. The thermally insulating coating technique is described as involving either spraying or plasma spraying and it is stated that the coating must be deposited after each contact with the melt. Drawbacks of the coating technique are stated to include defects resulting from non-uniform coating and surface contamination of the product as a result of particle pickup.
The mechanically roughened chill surface technique described in U.S. Pat. No. 4,250,950 is stated to enable the regulation of heat transfer by controlling the contact area between the chill and melt. The chill is roughened by shot peening or by grooving. While grooving is described in U.S. Pat. No. 3,345,738, supra, as ineffective for the process, it is noted that that early patent employed groove spacings of 0.25 inches (6.35 mm) whereas the current practices described in U.S. Pat. No. 4,250,950 considers groove spacings greater than 0.5 mm (0.02 inches) as a coarse grooving. The function of the grooving employed in the prior art described in U.S. Pat. No. 4,250,950 and the cross grooving of that patent is to avoid trapping evolving gases and/or air between the surface of a solidifying melt and the chill, which gases tend to produce uneven release and non-uniform heat transfer when trapped between a forming strip and the chill. This function of a roughened or grooved chill surface is believed to be generally understood and accepted in the art.
It is also known to continuously abraid a chill to continuously present a clean, matte metal surface into contact with the melt. For example, rotary wire brushes have been employed to contact and clean a cylindrical chill surface following release of the cast strip and before the surface again contacts the melt, as described, for example, by Huang and Fiedler in Metallurgical Transactions A, Vol. 12A, pgs. 1107-1112 June 1981. While this technique can enhance the quality of product produced either on a smooth or roughened, e.g., grooved, chill by providing a more uniform heat transfer, the process has not been entirely satisfactory. Further, the rapid abrasion of the chill, whether formed from high strength steel or a softer material such as copper or aluminum, by a continuously driven stainless steel brush or other aggressive abrading means requires frequent shutdowns for chill roll surface refinishing or roll changing.
As is known, even though a completely clean matte chill surface is provided at the beginning of a direct strip casting process, the high temperature of the melt contacting the chill results in rapid oxidation of the chill surface. This is particularly true where, as is usual, the chill moves in the open atmosphere for a substantial distance after release of the cast strip and before the chill surface is again presented in contact with the melt. In addition to oxides of the base metal of the chill, the natural oxide layer or coating which builds up on the chill may include at least a small percentage of oxides of the melt.
It has long been recognized that an accumulation or buildup of oxides and other material on the chill surface reduces the heat transfer rate between the melt and chill, and it is for this reason that some processes have provided an abrading means to continuously remove such material.
The natural oxide coating which inherently builds up on the chill surface during strip casting is not a homogenous, even coating and consequently tends to produce an uneven heat transfer rate depending upon the thickness and condition of the coating. It has been learned, also, that this coating is not a coherent, dense coating throughout but rather it builds as a dense, compact inner layer and a less dense outer portion comprising loose particles which can flake off or adhere to the surface of a strip cast on the chill.
Despite the long recognition that uniform heat transfer between the melt and chill surface is essential in a continuous or direct strip casting operation to avoid surface defects and thickness variations, and despite the substantial effort which has been directed to achieving such uniform heat transfer, the prior art processes are generally incapable of achieving such uniform heat transfer to the extent desirable in a commercial continuous or direct strip casting operaton. Accordingly, it is an object of the present invention to provide an improved method of directly casting metal strip by providing a more uniform heat transfer between a melt of the metal being cast and a chill surface moving in contact with the melt to continuously withdraw the strip
Another object is to provide an improved process for controlling the gauge thickness of metal strip produced by direct casting from a melt on a continuous chill moving in contact with the melt to withdraw the cast strip.
Another object is to provide an improved melt drag strip casting process employing a continuous chill surface to solidify and withdraw the strip from a melt of the metal to be cast and in which a thin compact natural oxide interface is established and maintained between the base metal of the chill and the melt to thereby control the rate of transfer of heat between the melt and the chill, the metal oxide interface being a compact, smooth layer of the natural oxide developed in the melt drag process and being firmly adhered to the base metal of the chill surface.
The foregoing and other features and advantages are achieved in accordance with the present invention wherein a melt of the metal to be cast is brought into contact with a continuously moving chill to solidify a strip of uniform thickness on the surface of the chill and the strip removed after solidification is substantially complete. The chill may be an internally cooled cylindrical casting wheel and the process will be described herein with specific reference to a process using such a chill, it being understood that other chill configurations such as a flat belt or so-called caterpillar track chill surface may also be employed.
In accordance with the invention, a melt of the metal to be cast is brought into contact with the chill which is driven at a predetermined rate to solidify and withdraw the strip from the melt at a substantially constant velocity. The invention will be described herein with reference to a process wherein the chill is positioned to effectively form one wall of a container for the melt, it being understood that other arrangements could be employed as suggested above. For example, the chill could be partially submerged into the top surface of the melt, or the chill may be positioned adjacent the edge surface of the melt container and the molten metal caused to overflow to contact the chill. In another possible arrangement, a conduit or nozzle could be employed to conduct the melt from a supply into contact with the moving surface of the chill.
Heat transfer from the melt to the chill is achieved at a uniform, desired rate while maintaining an oxide coating on the chill surface This is accomplished by establishing a coherent, dense, smooth natural oxide interface between the melt and the base metal of the chill, with the oxide interface consisting primarily of an oxide of the base metal of the chill surface and oxides of the primary metal and alloy metals of the melt. The uniform interface is achieved by continuously wiping or brushing the chill surface following release of a cast strip and prior to the surface re-entering the melt, whereby loose oxide particles and any adhering particles of the melt are continuously removed from the chill. The brushing or wiping action acts to lightly polish the oxide coating, producing a smooth surface on the oxide interface, with the oxide being of substantially uniform density over the entire chill surface. Further, this light polishing action can maintain the oxide thickness within a range which will produce a substantially uniform heat transfer rate. It is critical, however, that the polishing action not be sufficiently abrasive to remove the dense adhering oxide interface and thereby expose and abrade or damage the base metal of the chill surface. By controlling the polishing action to increase or decrease the oxide coating thickness and to maintain the desired thickness, the heat transfer rate can be controlled and thereby an effective strip gauge control is achieved.